1. Field of the Invention
This invention relates to particle beam projection systems and more particularly to calibration of such systems.
2. Description of Related Art
See U.S. Pat. No. 5,763,894 of Enichen et al. for "Calibration Patterns and Techniques for Charged Particle Projection Lithography Systems" and U.S. Pat. No. 5,283,440 Sohda et al. for "Electron-Beam Writing System Used in a Cell Projection Method".
In an EBPS (Electron-Beam Projection System) the size and orientation (rotation) of the projected cell or subfield must be adjusted in order for subfields or cells being formed to butt against the neighboring subfields or cells without gaps or an overlap between subfields or cells.
Previous calibration methods described in U.S. Pat. No. 5,763,894, supra, and U.S. Pat. No. 5,283,440, supra, have done this adjustment task by projecting an image formed by the reticle onto a matching target in the wafer plane, and the projected image is scanned over the matching target. The degree of matching is evaluated by measuring the peak signal strength of the backscattered electron signal or by measuring the width of the backscattered electron signal at a given threshold. The lenses that control the magnification and rotation of the subfields are stepped through a range of values and data is collected from the backscattered electron detector. The optimum adjustment is determined by the lens setting that maximizes the backscattered electron signal or minimizes the backscattered electron signal width.
The problem with the techniques of U.S. Pat. No. 5,763,894, supra, and U.S. Pat. No. 5,283,440, supra, is that for a single measurement it is impossible to determine the direction in which to make an adjustment. It is necessary to make enough trial adjustments in order to establish a maximum backscattered electron signal or a minimum backscattered electron signal width. A single measurement yields no information on the degree to which the adjustment has approached proximity to an optimum adjustment or in what direction an optimum can be approached.